This invention relates to a metal detector in particular for the detection of ferromagnetic or electrically conductive foreign matter in foodstuffs and the like.
Metal detectors have been used for many years to screen foodstuffs for the presence of foreign ferromagnetic or electrically conductive objects (usually metallic) which have been introduced during processing or packaging of the food.
One such arrangement is known as an inductive balance metal detector. Here, a central excitation coil is arranged coaxially with a pair of receiver coils. The three coils are typically coaxial and of similar diameter. The receiver coils are spaced equidistantly on either side of the excitation coil, typically with a spacing of 30–40 mm between the excitation coil and each receiver coil. A highly stable, pure sinewave having a typical frequency between 30 kHz and 1 MHz is applied to the excitation coil and the magnetic field thus generated results in a flux which links each receiver coil equally. The two receiver coils are connected to one another in series opposition so that the induced voltages (strictly, the induced e.m.f.'s) in each cancel out and a net zero output signal is in principle obtained.
Any ferromagnetic or electrically conductive material adjacent to this arrangement causes an imbalance in the flux linking one or other of the receiver coils, so that a voltage of non-zero amplitude is generated in them. This principle is used to allow detection of foreign objects in foodstuffs and the like, by moving the foodstuffs through the coils on a conveyor for example, and looking for a net voltage induced in the receiver coils.
One problem with this arrangement, however, is that both electrically conductive and ferromagnetic objects affect the flux linking the receiver coils. Unfortunately, many foodstuffs to be monitored are electrically conductive. For example, chicken breasts contain a significant quantity of salt water which conducts electricity and thus the voltage measured by the receiver coils as the chicken breast moves through them will change even if there is no contaminant present. This in turn means that if, in fact, there is a contaminant present, its presence is usually masked by the “product signal”. The peak amplitude of a 1 mm diameter iron filing can be of the order of only 50 nV. It is therefore highly desirable that the voltages induced from sources other than the contaminant are reduced as nearly as possible to zero.
One technique that has been employed to address this makes use of the different phase angles produced by different materials. The phase angle is the angle between the phase of the sinewave generator or oscillator (taken as 0° for simplicity) and the phase of the voltage induced in the receiver coils. Such a phase angle is not a simple function of the material passing through the detector, but instead depends on several factors such as the size of the material, the frequency of the oscillator, and the type of material.
Nonetheless, the phase angle of a “wet” product such as a chicken breast is sufficiently different to that of a piece of iron or brass, say, that the phase angle can be used to phase compensate for (i.e. filter out) the product itself. Wet products typically have a phase angle between −20° and +20° whereas iron has a phase angle around 100° and brass has a phase angle of around 70°. Thus by filtering the voltage measured at the receiver coils over −20° to +20°, the amplitude of the voltage output of the detector should remain around zero unless a contaminant is present. Having a different phase angle, the contaminant will cause an output voltage at the receiver coils which will not be filtered. This non-zero voltage can be used to trigger a rejection of the contaminated product from the conveyor carrying that product through the detector.
Even with this procedure, however, a residual voltage amplitude arises as a product passes through the detector whether or not it contains contaminants. This amplitude is broadly symmetrical and reaches a maximum as the product passes through the central excitation coil. This residual signal is often referred to as the bulk effect signal, as its magnitude is dependent upon the size or volume of the product itself.
Various attempts to remove the bulk effect signal and allow better sensitivity to small contaminants have been proposed. In one solution, a product passing a sensor on the conveyor triggers a counterbalancing signal of equal and opposite polarity to the bulk effect signal. This counterbalancing signal is electronically subtracted from the residual voltage measured after the product signal has been phased out.
Such a solution is only partly effective, however, as the bulk effect signal is dependent to a significant degree on the size and shape of the product itself: broadly speaking, larger products generate a bigger bulk effect signal. Thus, on a typical production line, with products whose size, shape and orientation to the detector are all different, a residual voltage often remains even with compensation. This can still be sufficient to mask any contaminant signal which, as explained, is typically extremely small.